The term “ionic liquid” refers to a salt composed solely of ions which generally has a melting point of 100° C. or below. Various applied research is being done on ionic liquids on account of their properties. In particular, given their non-volatility, flame retardancy and high heat resistance, research is even being conducted on the use of ionic liquids as lubricants (see, for example, Patent Documents 1 and 2, and Non-Patent Documents 1 and 2).
Ionic liquids have the physicochemical qualities required of a lubricant. In particular, fluorinated ion-containing ionic liquids such as tetrafluoroborate, hexafluorophosphate and bis(trifluoromethanesulfonyl)imide salts reportedly have excellent anti-wear properties. However, since ionic liquids contain halogen atoms, there are problems of high environmental load, especially concerning environmental problems at the time of leakage, waste disposal, and the like.
In addition, when a fluorine-containing ionic liquid is used as a lubricant, moisture infiltration is a problem. Even the infiltration of a trace amount of moisture sometimes leads to critical defects when this moisture reacts with hydrogen fluoride from fluorinated anions that have decomposed in chemical reactions (tribochemical reactions) activated by friction, causing metallic materials to corrode or polymer materials to deteriorate. Moreover, the hydrogen fluoride generated is strongly toxic and thus dangerous as well. Therefore, fluorine-containing ionic liquids generally have the reputation of being usable as lubricants only in space-related applications where moisture is absent or for ceramics without metal interfaces, and are not currently in practical use as general-purpose lubricants.